1. Any pill or tablet that is unusually or unreasonably large, such that it might be difficult for someone to swallow. The doctor gave me these horse pills to combat the infection, but just looking at them makes my throat hurt!

2. A story, statement, claim, proposal, etc., that is untrue or difficult to believe or accept (i.e., "hard to swallow"). Mike tried to feed us some horse pill about how he had inherited the money, but I'm pretty sure he stole it.